It is known to measure the level of the surface of a medium stored in a tank by using radar level gauges. Examples of such a device are given in U.S. Pat. No. 4,641,139. In a device of the type described in said patent specification, a tube that communicates with the medium in the tank is used as a waveguide for the microwaves that are transmitted by a radar unit and are received by the same radar unit following reflection from the surface of the medium in the tube.
Tubes that are used as waveguides in level measurement are usually circular and have a diameter that is at least twice as great as the wavelength of the microwaves that are used in the radar unit. When microwaves are transmitted in a tube with the given conditions, a plurality of different modes of microwaves is transmitted in the tube waveguide. To obtain the only desired microwave mode, a mode generator that comprises a waveguide is used in this case for generating a microwave mode of a higher order and in addition a long, cone-shaped antenna as a transition stage from the narrower waveguide to the wider tube in the tank for transmitting the desired mode in the tube waveguide.
The present invention presents an alternative design of an antenna, in which the design of the antenna renders a separate mode generator superfluous.
A known antenna structure for achieving a waveguide mode by means of a flat antenna is presented in patent specification EP 1 083 413.